1. Field of the Invention
The present invention relates to a process of producing a porous polypropylene hollow fiber or film.
2. Description of Prior Arts
A porous hollow fiber made of a polymer material having the surrounding wall with a large number of micropores formed therein is used widely as a device for separating various substances. For examples, in the clinical field, it is employable for separation of blood plasma or as a membrane for artificial lung and ultrafilteration.
As a process of producing a porous hollow fiber or a porous film, there .is known a process which comprises steps of preparing a hollow fiber or film made of a polymer material containing .a soluble material dispersed therein and then removing said soluble material by dissolving it with an appropriate solvent to form a large number of micropores in the surrounding wall of said fiber or in said film. Recently, an alternative process using a hollow fiber or film of a thermoplastic crystalline polymer material has been proposed and now is used in practice. In this process, the hollow fiber or film is heat-treated and then subjected to a drawing process to form pores in the surrounding wall of the fiber or in the film. Thermoplastic polymers which can be used for the above purpose are polyolefin, polyamide, polyester and other analogous copolymers. Among those materials, polypropylene (homopolymer of propylene or a copolymer of propylene and other monomer) is considered to be satisfactory as the polymer material for producing a porous hollow fiber or film because it is excellent in moldability, mechanical strength and resistance to chemicals.
Porous hollow fibers using polypropylene as the polymer material and processes of producing the same are described in Japanese Patent Provisional Publications No. 52(1977)-15627 No. 52(1977)-137026, No. 53(1978)-38715, 54(1979)-34418, No. 54(1979)-68414, No. 54(1979)-38623, No. 55(1980)-1314, No. 55(1980)-107505 and No. 57(1982)-5914. Most of these processes disclosed in the above publications comprises a procedure in which a polypropylene hollow fiber which has been spun is first heat-treated and then drawn at a temperature in the range of from a glass transition point up to 110.degree. C. (mainly at a temperature in the vicinity of room temperature) to form micropores whereby producing a porous body. The resultant body is again drawn at a temperature of not lower than 110.degree. C. and then further heat-treated so as to have the produced micropores fixed therein.
Porous polypropylene films and processes of producing the same are described, for instance, in Japanese Patent Publications No. 46(1971)-40119, No. 55(1980)-2176 ad No. 55(1980)-32531. Most of these processes disclosed in the above publications comprises a procedure in which a moulded polypropylene film is first heat-treated ad then drawn at a temperature in the vicinity of room temperature or at a temperature of not lower than the second order transition point of the thermoplastic resin employed to form micropores producing a porous body. The resultant body is again drawn at a temperature higher than that of the first drawing process ad then further heat-treated so as to have the produced micropores fixed therein.
In the conventional process for producing the polypropylene hollow fiber, the process comprises two drawing steps consisting of drawing the fiber at a temperature in the vicinity of room temperature and again drawing it at a temperature higher than that of the first drawing (not lower than 110.degree. C.), so that the process is complicated.
Further, in the above conventional process, the step of increasing the crystallinity in a polypropylene hollow fiber is necessarily introduced before the drawing step in order to improve the quality of resulting porous polypropylene hollow fiber. Therefore, the manufacturing process as a whole is very complicated.
Still further, the porous polypropylene hollow fiber obtained in the conventional .process is not always satisfactory in the use for separation of blood plasma, etc. which requires a relatively large pore size.
The process of producing a porous polypropylene hollow fiber by drawing at a temperature of not higher than 110.degree. C. (mainly at a temperature in the vicinity of room temperature) can only give porous polypropylene hollow fiber having micropores with relatively small pore sizes and a low porosity, though the process involves simple steps.
In view of the application purpose of a porous polypropylene film, micropores formed in the film are desired to be uniformly distributed at a porosity as desired. However, the porous polypropylene film obtained in the above process has a relatively small average pore size such as not larger than 5,000 angstrom and a low porosity. For these reasons, the porous polypropylene film obtained in the above conventional process is not appropriate in the use as membrane for separation of blood plasma, etc., and the use of the porous film is thus under limitation.
The conventional process of producing a porous polypropylene film generally requires a supplemental procedure for increasing the crystallinity in order to improve the quality of the resulting porous polypropylene film. Therefore, the process of producing a porous polypropylene film comprising such a supplemental procedure tends to be complicated as a whole.